I'd be surprised if re-using the crush washer "as-is" could be proven to cause any issues.

Think of all the cassettes in the world, ever taken off and put back on: they all re-use their crush washer. Including all of mine - and I've done hundreds (thousands?) of them if you count the ones I did when I worked in a shop.

Except the cassettes that don't come with a crush washer - then the lockring goes on directly against the last cog. Still no issues.

So, no need to flatten it (hammer or otherwise), just put it on and tighten it up. If in doubt tighten it with a torque wrench. The spec is printed on the lock ring and the number is pretty high, almost as tight as BB cups and cranks IIRC.

On the other hand, I don't know why your previous installation might have seemed loose. Speculation here: a bit of dirt or something was there, then fell out... nah, that doesn't seem likely. But at any rate, if you simply re-use your crush washer when you reinstall your cassette it should be fine.